The present invention relates to an objective lens which is installed in a device employing multiple types of light beams having different wavelengths, such as an optical information recording/reproducing device for recording information to and/or reproducing information from multiple types of optical discs differing in recording density.
There exist various standards of optical discs (CD, DVD, etc.) differing in recording density, protective layer thickness, etc. Meanwhile, new-standard optical discs (HD DVD (High-Definition DVD), BD (Blu-ray Disc), etc.), having still higher recording density than DVD, are being brought into practical use in recent years to realize still higher information storage capacity. The protective layer thickness of such a new-standard optical disc is substantially equal to or less than that of DVD. In consideration of user convenience with such optical discs according to multiple standards, the optical information recording/reproducing devices (more specifically, objective lenses installed in the devices) of recent years are required to have compatibility with the above three types of optical discs. Incidentally, in this specification, the “optical information recording/reproducing devices” include devices for both information reproducing and information recording, devices exclusively for information reproducing, and devices exclusively for information recording. The above “compatibility” means that the optical information recording/reproducing device ensures the information reproducing and/or information recording with no need of component replacement even when the optical disc being used is switched.
In order to provide an optical information recording/reproducing device with the compatibility with optical discs of multiple standards, the device has to be configured to be capable of forming a beam spot suitable for the particular recording density of the new disc (in the switching of the optical disc to the new disc of a different standard) by changing a NA (Numerical Aperture) of the light beam employed for the information reproducing/registering, while also correcting spherical aberration which varies depending on the protective layer thickness. Since the diameter of the beam spot can generally be made smaller as the wavelength of the beam gets shorter, multiple laser beams having different wavelengths are selectively used by the optical information recording/reproducing device depending on the recording density of the optical disc being used. For example, for DVDs, a laser beam with a wavelength of approximately 660 nm (shorter than approximately 790 nm for CDs) is used. For the aforementioned new-standard optical discs, a laser beam with a wavelength still shorter than that for DVDs (e.g. so-called “blue laser” around 408 nm) is used in order to deal with the extra-high recording density.
An objective lens having the compatibility with optical discs with multiple standards is disclosed, for example, in Japanese Patent Provisional Publications Nos. 2002-6210 (hereafter, referred to as JP 2002-6210A), 2004-288346 (hereafter, referred to as JP 2004-288346A), and 2005-513701 (hereafter, referred to as JP 2005-513701A). JP 2002-6210A discloses a cemented objective lens formed of two types of lenses which have optical characteristics different from each other and are cemented to each other. In an optical system disclosed in JP 2002-6210A, the compatibility with two-types of optical discs (i.e., CD and DVD) is achieved by selectively using a near-infrared laser beam and a red laser beam and by causing each of the laser beams to enter the objective lens as a collimated beam.
JP 2004-288346A discloses a single element objective lens having a diffracting structure on a surface thereof. More specifically, the diffracting structure of the objective lens is configured such that a ratio between diffraction orders at which the diffraction efficiencies of a blue laser beam, a red laser beam and a near-infrared laser beam take the respective maximum values is represented as “2:1:1”. In an optical system of JP 2004-288346A, each of a blue laser beam and a red laser beam enters the objective lens as a collimated beam, while a near-infrared laser beam enters the objective lens as a diverging beam. As a result, the compatibility with three types of optical discs is achieved.
JP 2005-513701A discloses a cemented lens having compatibility with two types of optical discs (i.e., DVD and HD DVD).
However, the cemented objective lens disclosed in JP 2002-6210A has a drawback that the cemented objective lens is not able to support reading or reproducing operation for a new standard optical disc having higher recording density than those of the CD and DVD although the cemented objective lens can be used suitably for the CD and DVD.
The objective lens disclosed in 2004-288346A has a drawback that the diffracting structure is not able to suitably correct the spherical aberration for both of the blue laser beam and the near-infrared laser beam. Since in general the tolerance to aberration decreases in accordance with increase of the recording density, the diffracting structure is designed such that the spherical aberration caused when the blue laser beam is used is corrected more suitably relative to the spherical aberration caused when the near-infrared laser beam is used. In order to suitably correct the spherical aberration caused when the near-infrared laser beam is used, it becomes necessary to increase the degree of divergence of the near-infrared laser beam entering the objective lens to some extent. In this case, a relatively large amount of off-axis aberration, such as a comma, may be produced when the lens shifts for a tracking operation.
The cemented objective lens disclosed in JP 2005-513701A is configured to have a substantially spherical cementing surface. Therefore, the function of correcting the spherical aberration of the cemented objective lens is limited. More specifically, if the cemented objective lens is designed to suitably correct aberration caused when one of the DVD and HD DVD is used, aberration caused when the other of the DVD and HD DVD is used can not be corrected sufficiently.